Fix IOTS cleanup on non Windows

[mconnew]

With recent changes to improve performance on Windows by bringing back use of IO completion ports, a bug was introduced on non-Windows platforms where cleaning up an IOThreadScheduler resulted in an exception due to attempting to free a null overlapped.

[mconnew]

The errors in the failure make absolutely no sense:

---ETW Trace for Test SecutityBindingElementTest.Property_DefaultAlgorithmSuite Begins---
SecutityBindingElementTest.Property_DefaultAlgorithmSuite: Throwing an exception. Source: System.ServiceModel 4.0.0.0. Exception details: System.ArgumentNullException: Value cannot be null. (Parameter 'tokenXml')
---ETW Trace for Test SecutityBindingElementTest.Property_DefaultAlgorithmSuite Ends---

    SecutityBindingElementTest.Property_DefaultAlgorithmSuite [FAIL]
      Assert.Equal() Failure
      Expected: Basic256
      Actual:   Basic256
      Stack Trace:
        /_/src/System.ServiceModel.Security/tests/ServiceModel/SecutityBindingElementTest.cs(36,0): at SecutityBindingElementTest.Property_DefaultAlgorithmSuite()
  Finished:    System.ServiceModel.Security.Tests

It's saying it failed because Basic256 doesn't equal Basic256. It's also picking up ETW exception trace from some completely different place. I'm going to rerun the failed test to see if that fixes it as this looks like a JIT issue.

[mconnew]

/azp run

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